Electrodes are known comprising a current collector supporting a material capable of releasing a chemical species that functions as a charge carrier. One example of such an electrode is a secondary cell electrode comprising a metal collector supporting a material (active material) capable of reversibly storing and releasing such a chemical species. This electrode can be used favorably as the positive or negative electrode of a lithium ion battery that is charged and discharged by means of the movement of lithium ions to and from an electrolyte (typically a nonaqueous electrolyte) interposed between this electrode and a counter-electrode. A typical method of supporting the active material on the collector is to apply to the electrode collector a paste or slurry composition (active material composition) comprising a powder of an active material dispersed in a solvent to thereby form a layer of the active material (active material layer). From the standpoint of reducing environmental impact, reducing material costs, simplifying the equipment, reducing waste volume and improving the handling properties and the like, the active material composition used in this method is preferably an aqueous composition, in which the solvent constituting the above medium (dispersion medium for the active material powder or the like) is an aqueous solvent.
Depending on the makeup of the active material, however, using an aqueous composition may lead to problems of decreased battery capacity or reduced discharge characteristics due to increased initial internal resistance. This may occur because the active material reacts with water contained in the paste. For example, when a lithium-nickel oxide or other lithium-transition metal oxide (here and below, an oxide containing lithium and one or two or more transition metal elements as constituent metal elements) is used as a positive-electrode active material, an exchange reaction occurs between lithium ions and protons on the surface of the positive-electrode active material dispersed in an aqueous solvent, which may raise the pH value (that is, the alkalinity) of the aqueous active material composition. When this high-pH aqueous active material composition is applied to a positive-electrode collector (for example, an aluminum positive-electrode collector), compounds exhibiting high electrical resistance (for example, oxides or hydroxides) are more likely to occur on the surface of the collector. Production of such high-electrical-resistance compounds can cause increased initial internal resistance in the battery (thereby interfering with high output).
In this regard, Patent Document 1 describes a technique for avoiding the phenomenon of high-electrical-resistance compounds occurring during formation of an active material layer by application of an aqueous active material composition by providing a layer containing an organic solvent-soluble polymer (binder) and a conductive material (conductive layer) on the surface of the collector, and using this layer as a barrier layer to block direct contact between the collector and water.
Patent Document 1: Japanese Patent Application Laid-open No. 2006-4739
Such a barrier layer must have both water-resistance (the property of blocking direct contact between water and the collector to prevent production of high-electrical-resistance compounds) and electrical conductivity (in other words, the property of not excessively raising the resistance between the active material layer and the collector layer). However, these two properties normally conflict with each other. For example, increasing the percentage content of the conductive material in the barrier layer is an effective way of improving the conductivity of an electrode having this barrier layer, but merely increasing this percentage will tend to reduce the water resistance of the barrier layer because the percentage content of the binder is reduced proportionally. Conversely, increasing the percentage content of the binder in order to improve the water resistance of the barrier layer tends to reduce the electrical conductivity because the content of the conductive material is reduced proportionally.